Ultra low temperature thermal regenerator

ABSTRACT

Temperatures below 10* K can be achieved operating on a GiffordMcMahon cycle by the utilization of a thermal regenerator having a pressure-regulated helium reservoir positioned within an annulus circumferentially disposed about a helium refrigerant flow channel. Thermal conduction between the cyclically flowing helium refrigerant and the helium reservoir is effected by a plurality of porous metallic plates disposed at a perpendicular attitude relative to the refrigerant flow stream with adjacent plates being separated by thermal insulators, e.g. plastic rings, bonded to the plates to inhibit longitudinal thermal conduction in the regenerator. Preferably the bonded rings are concentrically disposed to form external sidewalls defining the helium refrigerant flow channel and to contain the helium reservoir. Because the helium heat reservoir is situated at a location remote from the channel wherein the helium refrigerant flows, the heat capacity of the reservoir can be varied by adding or removing helium from the reservoir without adversely effecting the helium refrigerant flow through the regenerator.

United States Patent Nesbitt et al.

[54] ULTRA LOW TEMPERATURE THERMAL REGENERATOR [72] Inventors: Loyd B. Nesbitt; Robert B. Fleming,

both of Scotia, NY.

[73] Assignee: General Electric Company [22] Filed: June 20, 1968 [211 Appl. No.: 738,535

[52] US. Cl. ....l65/10, 165/32, 62/6, 60/24 [51] Int. Cl ..F28d 17/00 [58] Field of Search ..165/2, 10, 31, 32, 39; 62/6; 60/24 [56] References Cited UNITED STATES PATENTS 2,879,976 3/1959 Rose ..165/39 X 2,906,101 9/1959 McMahon et a1. ..62/6 3,188,818 6/1965 Hogan ..62/6 3,270,802 9/ 1966 Lindberg ..165/96 3,375,867 4/1968 Daunt ..62/6 3,397,738 8/1968 Daunt ..62/6

Primary ExaminerSamuel W. Engle Att0rneyJ0hn F. Ahern, Paul A. Frank, Julius J 51 Sept. 19,1972

Zaskalichy, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [5 7] ABSTRACT Temperatures below 10 K can be achieved operating on a Gifford-McMahon cycle by the utilization of a thermal regenerator having a pressure-regulated helium reservoir positioned within an annulus circum ferentially disposed about a helium refrigerant flow channel. Thermal conduction between the cyclically flowing helium refrigerant and the helium reservoir is effected by a plurality of porous metallic plates disposed at a perpendicular attitude relative to the refrigerant flow stream with adjacent plates being separated by thermal insulators, e.g. plastic rings, bonded to the plates to inhibit longitudinal thermal conduction in the regenerator. Preferably the bonded rings are concentrically disposed to form external sidewalls defining the helium refrigerant flow channel and to contain the helium reservoir. Because the helium heat reservoir is situated at a location remote from the channel wherein the helium refrigerant flows, the

heat capacity of the reservoir can be varied by adding or removing helium from the reservoir without adversely effecting the helium refrigerant flow through the regenerator.

11 Claims, 7 Drawing Figures ULTRA LOW TEMPERATURE THERMAL REGENERATOR This invention relates to thermal regenerators for use in cryogenic refrigerators of the Gifford-McMahon or Stirling variety and in particular to a regenerator utilizing a pressure controlled helium heat reservoir positioned at a location remote from the cyclic flow stream of the refrigerant. The invention herein described was made in the course of or under a contract or subcontract thereunder with the department of the Air Force.

Cyclically operating thermal regenerators of the Gifford-McMahon or Stirling variety generally include a packing material which receives heat from a relatively high pressure fluid stream flowing in a first direction and stores the heat for a short period, e.g., usually less than one second, before transferring the heat back to a lower pressure fluid stream flowing in the opposite direction. Use of a Gifford-McMahon or Stirling refrigerator at temperatures below about K, however, generally has been limited because the heat capacity of conventional packing materials, such as lead, decreases rapidly at cryogenic temperatures thereby severely restricting the ability of the packing material to function properly as a heat storage medium. In attempts to reduce the minimum permissable operating temperatures of cyclically operating regenerators, the heat capacity of helium gas, which has the highest heat capacity of any known substance belowabout 10 K, heretofore has been employed with limited success. For example, it has been proposed that small diameter helium-filled sealed tubing be positioned within the flow channel .of a helium refrigerant stream to serve as a heatreservoir as the helium refrigerant is cyclically surged through the channel. At reduced temperatures however, the helium pressure within the sealed tubes is reduced thereby substantially limiting the heat capacity of each tube for a given volume coil in the flow chamber. Similarly, because the coils are positioned within the flow channel of the refrigerant, the heat capacity of the helium packing coils cannot be varied, e.g., by an increase in the number of coils or coil volume, without adversely effecting the void volume in the refrigerant flow channel, and the packing coils often produce a poor flow distribution in the refrigerant passing therethrough. Furthermore, thermal contact between the flowing refrigerant and the helium within the packing coils is necessarily limited by coil configurations required to effect maximum helium storage within a minimum volume of the refrigerant flow channel.

It is therefore an object of this invention to provide an ultra-low temperature thermal regenerator having an enlarged heat transfer surface in contact with the refrigerant stream.

It is also an object of this invention to provide an ultra-low temperature thermal regenerator having a refrigerant flow independent of the thermal capacity of the heat reservoir employed therein.

It is a further object of this invention to provide a thermal regenerator capable of high efficiency operation at temperatures below 16 K and having limited heat transfer in the longitudinal direction of the refrigerant flow channel.

These and other objects of this invention are accomplished in a thermal regenerator for operation in a Gifford-McMahon or Stirling refrigerator by positioning a helium reservoir at a location remote from the refrigerant flow stream and by regulating the helium pressure within the reservoir to control the heat capacity of the regenerator reservoir. Thus a cyclically operating thermal regenerator in accordancewith this invention generally is characterized by a channel for the cyclic flow of a fluid refrigerant therethrough and a remotely situated helium reservoir thermally linked at intermittent intervals to the refrigerant flow stream to sequentially receive, store and transmit heat to the cyclically flowing refrigerant. Heat is transmitted between the refrigerant and the reservoir preferably by a plurality of spaced apart, high heat conductivity metallic plates which form a porous obstruction within the refrigerant flow channel and extend into the helium reservoir to reduce thermal gradients between the flowing refrigerant and the helium reservoir. External means also are provided for controlling the pressure of helium within the regenerator by a variation in the quantity of helium in the reservoir as the temperature of helium in the reservoir changes.

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof may best be understood by reference to the following description, taken in connection with the accompanying drawings, in which:

FIG. I is a cutaway isometric view of a thermal regenerator constructed in accordance with this invention,

FIG. 2 is a plan view of the thermal regenerator of FIG. 1,

FIG. 3 is a sectional view taken of the thermal regenerator along lines 3-3 of FIG. 2,

FIG. 4 is a simplified partially sectionalized illustration of the thermal regenerator of thisinvention connected for operation on the coldest stage of a Gifford- McMahon cycle,

FIG. 5 is a graph depicting the low temperature heat capacity of lead spheres and helium at constant pressure, for a particular regenerator design,

FIG. 6 is an enlarged portrayal of the perforated thermal conductors employed in the regenerator, and

FIG. 7 is an isometric view of an alternate thermal regenerator construction in accordance with this invention. V

A preferred thermal regenerator l0 constructed in accordance with this invention is depicted in FIGS. 1-3 and generally includes a centrally disposed channel 12 for the cyclic flow of a helium refrigerant therethrough with a relatively stagnant helium reservoir 13 being confined within an annulus l4 circumferentially disposed about the central channel. to serve as a matrix material for sequentially receiving, storing and returning heat to the flowing refrigerant. A plurality of porous plates 16 are situated at a perpendicular attitude relative to the cyclic flow direction of the thermal refrigerant within channel 12 (as shown by dual headed directional arrow 17) and function as hear conductors during regenerator operation to rapidly reduce any thermal gradient existing between the helium reservoir and the flowing refrigerant. Thus the porous plates should be of a material having a high thermal diffusivity, i.e., the thermal time constant (the'time required to effect about two-thirds the total temperature change at the center of the plate in response to a temperature change at the periphery of the plate) should be a small fraction, e.g., less than one-half of the thermal cycle period. To inhibit heat conduction in the direction of refrigerant flow, e.g., along the longitudinal axis of the regenerator, adjacent plates 16 within the thermal regenerator are separated by concentrically disposed insulating gaskets 18 and 20 with interiorly positioned gaskets 18 functioning to form a sidewall separating the helium refrigerant from helium reservoir 13 thereby defining flow channel 12 for the helium refrigerant while externally disposed gaskets 20 serve to form an exterior sidewall for the helium reservoir within annulus 14. Preferably, annulus 14 has a volume considerably larger than, e.g., at least eight-fold, the volume of central channel 12 to assure a sufficient heat capacity in helium reservoir 13 during regenerator operation. While the thermal regenerator of this invention preferably is disposed within a vacuum during operation and ideally needs no exterior insulation for thermal isolation, an outer sheathing 21 of plastic coat ing the exterior sidewall of the regenerator can serve as a sidewall mechanical support. Top and bottom plates 22 and 23 having centrally disposed apertures 24 and 25, respectively, for the passage of helium refrigerant therethrough serve to generally seal the ends of annulus 14 thereby defining the volume of helium capable of being stored by the regenerator.

A The interior of annulus 14 is connected to an external source of helium 26 through tubing 27 inserted in aperture 28 within top plate 22 to permit external control of the pressure within the annulus and a pressure gauge 29 is positioned along the length of tubing 27 to permit continuous visual monitoring of the helium reservoir pressure. For automatic regulation of the pressure within annulus 14 to a tollerable lever, e.g., preferably above atmospheres, a suitable sensor 30 is fixedly secured within top plate 23 and extends into helium reservoir 13 to automatically control the operation of value 32 within tubing 27. Thus external source 26 permits the addition of helium to annulus 14 as the regenerator cools during operation thereby maintaining a reservoir pressure above 5 atmospheres notwithstanding a reduction in the reservoir temperature below 16 K to assure a high heat storage capacity (which capacity is dependent upon helium pressure) for a regenerator of a fixed volume. Suitable means, such as relief valve 34 in tubing 27, serves to exhaust gas from the helium reservoir upon a heating of the helium thereby avoiding a 60-fold increase in pressure within annulus 14 which would occur if a fixed volume of helium were warmed to room temperature from a temperature of 8 K at a pressure of atmospheres. Valve 32 and relief valve 34 generally do not admit or remove helium from reservoir 13 during pressure fluctuations associated with each thermal cycle of the regenerator but rather respond only after a prolonged period, e.g., a period preferably of three cycles or more, of reservoir pressure outside a set pressure range. Because plates 16 are porous within annulus 14, a single connection to the annulus through aperture 28 permits pressure control over the entire length of the annulus.

The mode of operation of the thermal regenerator of this invention preferably is in a Gifford-McMahon, or

Stirling, refrigerator such as is shown in simplified form in FIG. 4 wherein thermal regenerator 10 is connected through a cross-over tube 36 to an expansion chamber 38. While FIG. 4 shows only a single, i.e., the coldest regenerator and piston of the refrigerator, in actual practice a plurality of such stages would be cascaded to obtain temperatures below l6 K. A piston 39 is axially disposed within chamber 38 and suitable means (not shown) are connected to the upper end of the piston to reciprocate the piston within the chamber at a convenient rate generally between I00 and 200 strokes per minute. In the operation of regenerator 10, high pressure helium refrigerant is admitted to the regenerator through conduit 49 connected to channel 12 of the regenerator and piston 39 is raised approximately onefourth the length of its stroke within chamber 38 whereupon admission of high pressure gas is terminated and the system is sealed. The admission of the high pressure helium refrigerant to channel 12 also effects a heat transfer from the refrigerant to helium reservoir 13 through porous plates 16 to reduce any thermal gradients therebetween. Further upward motion of piston 39 causes a reduction in pressure of the closed system, and a consequent cooling of the gas by adiabatic expansion. The system then is opened above the regenerator and piston 39 is returned to a downward position to exhaust the cooled helium refrigerant through conduit 49. As the cooled helium flows through the central channel 12 of the regenerator, the heat which had been stored by the helium reservoir 13 within annulus 14 during the admission of the high pressure gas to the regenerator is returned to the expanded helium refrigerant gas and the helium reservoir is further cooled by heat conductivity through porous plates 16 to effect an approximately zero thermal gradient between the expanded exhaust helium refrigerant and the helium reservoir. Upon exhaust of the expanded helium refrigerant from the regenerator, high pressure gas is readmitted to regenerator 10 to repeat the cooling cycle. Thus the temperature of the helium reservoir of the regenerator is incrementally reduced with each cycle until the portion of the reservoir proximate bottom plate 23 reaches a desired lower limit of approximately 6 K. Because a reduction in the helium reservoir temperature tends to lower the reservoir pressure, helium is intermittently leaked into reser voir 13 from source 26 during operation to maintain the pressure of the reservoir at a suitable level of about 10 atmospheres notwithstanding the reduced temperature of the reservoir.

Because the helium within annulus 14 of thermal regenerator 10 must accept heat from the high pressure gas during the initial portion of each cycle and retain the heat until the expanded low pressure gas passes in the opposite direction through channel 12 of the regenerator, a high heat capacity per unit length of the regenerator is desirable to limit the size of the regenerator and the heat losses associated therewith. As will be noted from FIG. 5 wherein the heat capacity per unit length of both helium at 10 atmospheres and lead is depicted, the thermal capacity of lead drops swiftly at temperatures below 16 K to an order of slightly above zero at 6 K generally negating the utilization of conventional lead packed regenerators at temperatures below l6 K. Helium at constant pressure however shows a generally rising heat capacity with decreasing temperatures and, in fact, shows an approximately two-foldincrease in heat capacity between 15 K and 8 K. Because the amount of heat capable of being retained by helium reservoir 13 is dependent upon the density ofthe helium gas within annulus 14,

the heat capacity of the heliumreservoir can be altered simply by an adjustment of valve 32 and/or relief valve 34 to vary the pressure of the heliumwithin annulus 14 without affecting the void volume in channel .12. Similarly, the heat capacity of helium reservoir 13 can be increased by an outward enlargement of annulus 14 thereby increasing the volume of helium stored within the annulus without producing an adverseeffect on the flow of refrigerant through central channel 12.

Because the heat capacity of randomly packed lead spheres is greater at temperatures above 16 K than the heat capacity of helium at approximately 10 atmospheres, the portion of conduit 49 proximate central channel 12 can be packed with lead spheres 54 to provide increased heat capacity at temperatures above 16 K with helium reservoir 13 within thermal regenerator l producing the majority of heat capacity at operating conditions below 10 K. Thus, at K where at the specific heat of helium at atmospheres pressure is about 2.4 joule/gK, as compared to about 0.0015 joulelgK for lead,.e.g. a ratio of 1,600 to 1, the helium stores -fold the quantity of heat stored by the lead per unit volume to effectively negate the effect of the 1 heat capacity of the lead spheres upon the regenerator. At temperatures of 16 K or higher, however, the lead spheres have an appreciable effect on the heat capacity of the regenerator and serve to rapidly reduce the regenerator to cryogenic temperatures.

The porous platesl6 employed in thermal regenerator 10 are shown in FIG. 6 and comprise two annular porouszonesSl and 52 for disposal in the refrigerant flow stream and helium reservoir, respectively. Those portions53 and 54 of plate 16 to be positioned intermediate insulating gaskets 18 and 20, respectively, preferably are unperforated to strengthen the bond between theplates andgaskets. Plate 16 generally can be of .any material having a high thermal conductivity and a low specific heat with any conventional method such as drilling, punching, etching or theuse of sintered matrices of spheres, chips, wires, etc. being suitable to obtain a porous structure through whichthe cyclically flowing helium refrigerant can pass. Aluminum, copper and sapphire generally are quite suitable as porous plate materials, although aluminum generally is preferred because aluminum is easily worked ,and bonds well to epoxy plastics. When aluminum is used to form the porous plates in regenerator 10, preferably insulating gaskets l8 and 20 are fabricated of aninorganic plastic which is bonded to the porous aluminum plates by the use of an epoxy or polyestheradhesive such as EPON resin No. 820 and EPON curing agent V-40. Because both the heliumwithin annulus .14 and insulating gaskets 18 and 20 do not conduct heat very rapidly, heat transfer in the direction of refrigerant flow is impeded and a thermal gradient of about 8 K may be formed between top plate22 and bottom plate 24 of regenerator 10 during operation. Preferably, the density of plates16 per unit length of regenerator 10 is high, e.g., greater then plates per cm, to minimize the length of'the regenerator.whileassuring maximum thermal contact between the cyclically flowing refrigerant andtherhelium reservoir.

Preferably the porous plates 16 exhibit low resistance 10 heat transfer at cryogenic temperatures,

e.g.,.at 10 K very pure aluminum .hasa thermal conductivity about 30-fold'thatat roomtemperaturewhile exhibiting very.little transient lag in the cyclical .conduction of heat, e.g., at 10.Kzthe heat capacity of aluminum is about 1/700 the room temperature value.

Thus, the;th.ermal diffusivity,.e.g.,thermalconductivity divided by theaproduct of the density and the specific heat, isincreasedby a factorof about.2 l ,000 relative to the thermal diffusivity of aluminum at room tempera- :ture and veryrapid temperature changes inrtheplates .canbe effected atcryogenic temperatures.

Plates 16 must be thick enough for mechanical strength, but thin enough .to provide sufficient .heat

transfer area. vGenerally plate,thicknessesin the range 0.1 .mm to 0.3 mm canbe employed. The thickness of the porousrplatesrand the, perfor.ated area of :theplates however generally will vary .with factors such .as the rate ofihelium refrigerant-flowthrough-channel .12 and the tolerablermechanicalstrength of plates :16 to permit ease of manufacturing. andmaterialhandling.

The perforations .55 in porous plates 16 preferably should :beof xhighrdensity and small diameter to maximizecthe'heat transfersurfacein contact-with the-flowing refrigerant through .channel 112. However .if the metal partitionsbetweenperforationsbecome too thin, .thermal flow'ina radial.directionthrough;plates l6can :bewimpeded therebyadversely affecting the high thermal diffusivity of .the metal plate. Plates having 3 grams persecondthrough channel 12. The40 percent open aluminum plates.also.arecharacterized by a uniformly .distributedheattransfer surface of 94 sq. centimeters per cubic .centimeter of helium in com- ;parison.to.aheat transfer'surface ofapproximately 5 .10 sq. centimeters-per cubic centimeter refrigerant :flowexhibitedby coilspositioned within the flow channel.

Although the portion '52 of perforated plate 16 within annulus 14 is shownas beingapertured,porosity is notessential in the stored helium volume. Porosity :however isradvantageous notonly because thestored volume of helium in annulus l4 is increased but also because of the increase in surfacearea contact between the'plates and .the-storedchelium. The latter advantage isespecially.noticeablewhen plates 16 are of substantial thickness, e.g., of .aithickness equal to or greater :than thechoie diameter. To further increase.the.heat transfer. characteristics betweenthe plates and the helium, conventional. techniques such asthe. employment :of fins, or a slight rippling in thealuminum plate can be .utilized.

.When it is desired to reduce the .distance along which heat must travel through plates 16 and thereby increase the operating speed of the regenerator, a thermal regenerator.58.of the type depicted in FIG. 7 having dual rectangular channels 62 and 64 for the cyclic flow of helium refrigerant can be employed. The helium reservoir for the reception, storage and return of heat from the cyclically flowing refrigerant during the regenerator cycle is positioned within sections 66, 67 and 68 to obtain close proximity to, while remaining outside, the refrigerant flow channels. A plurality of porous aluminum plates 69 are disposed at a perpendicular attitude relative to the direction of refrigerant flow within channels 62 and 64 with adjacent plates being thermally separated by and fixedly secured to plastic insulating gaskets 71 which gaskets also serve to define the sidewalls both of refrigerant flow channels 62 and 64 and helium reservoirs 66, 67 and 68. Ideally each of reservoirs 66, 67 and 68 are communicated with each other as well as an external helium source (not shown) to assure uniformity in the helium pressure (and therefore the heat capacity per unit volume) of each reservoir with central channel 67 preferably having a volume double the volume of end channels 66 and 68, e.g., double heat capacity, for maximum efficiency.

The regenerator portrayed in FIG. 7 functions in a manner identical to the operation of the regenerator of FIG. 1 except for the fact that the cyclically flowing refrigerant is divided into two channels 62 and 64 in regenerator 58 to reduce the heat transfer interval between the refrigerant and the surrounding reservoirs. It will be appreciated that any number of flow channels and reservoirs in any geometrical configuration can be employed in regenerators constructed in accordance with this invention provided the helium heat reservoir is positioned outside the refrigerant flow channel with heat transfer from the flowing refrigerant to the helium reservoir being effected by a heat conductor in contact with both mediums. We intend, therefore, by the appended claims, to cover all such modifications and changes as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In a cyclically operating thermal regenerator for operation at temperatures below 16 K wherein heat from a fluid stream flowing in a first direction is received and stored for a finite period by a helium reservoir prior to transfer of said heat to a fluid stream flowing in the opposite direction, the improvement comprising means for controlling the pressure of helium within said reservoir by varying the quantity of helium in said reservoir as the helium temperature changes within said reservoir.

2. A cyclically operating thermal regenerator according to claim 1 wherein said helium reservoir is situated within a housing externally located relative to said fluid stream flow channel, said thermal contact between said helium reservoir and said fluid streams being effectuated by a high thermal conductivity material protruding through the walls of said helium reservoir housing and extending into both said helium reservoir and said fluid streams flow channel.

3. A cyclically operating thermal regenerator according to claim 1 wherein the helium within said reservoir is at a pressure above 5 atmospheres and at a temperature of below 16 K.

4. A cyclically operating thermal regenerator according, to claim 2 wherein said high thermal conductivity material is selected from the group consisting of aluminum, copper, and sapphire, said material being a porous structure disposed substantially uniformly in said fluid streams flow channel to assure uniform distribution of flow at any cross-section of said flow channel.

5. A cyclically operating thermal regenerator according to claim 2 wherein said thermal contact between the reservoir and the flow streams is effectuated by a plurality of porous metallic plates spaced in insulated relationship with respect to one another within the path of said fluid streams at a generally perpendicular attitude to the direction of fluid flow.

'-6. A cyclically operating thermal regenerator for operation at temperatures below 16 K comprising a channel for the cyclic flow of a fluid refrigerant therethrough, a reservoir containing a gaseous medium, said reservoir being positioned adjacent said flow channel to sequentially receive, store and transmit heat to said fluid refrigerant during a complete flow cycle of said fluid refrigerant, means for controlling the pressure of the gaseous medium within said reservoir by varying the quantity of said medium in said reservoir as the temperature within said reservoir changes, and a plurality of spaced-apart, high conductivity metallic elements spaced in insulated relationship with respect to one another, said elements forming an apertured obstruction within said refrigerant flow stream and protruding through the sidewalls separating said flow channel from said reservoir to extend into said gaseous medium.

7. A cyclically operating thermal regenerator for operation at temperatures below 16 K comprising a channel for the cyclic flow of a refrigerant therethrough, a generally stagnant helium heat reser-' voir positioned adjacent said channel, and a plurality of porous, high heat conductivity elements disposed within said cyclic flow channel and extending into said helium reservoir to conduct heat between said reservoir and said refrigerant upon the existence of a thermal gradient between said helium and said refrigerant.

8. A cyclically operating thermal regenerator according to claim 7 wherein said refrigerant cyclically flows through a plurality of channels and said helium reservoir is disposed on opposite sides of each of said channels.

9. A cyclically operating thermal regenerator according to claim 7 wherein said heat conductivity elements are a plurality of porous metallic plates disposed at a perpendicular attitude relative to the refrigerant flow, adjacent of said plates being separated both by peripherally positioned thermal insulators and by interiorly positioned thermal insulators, said interiorly positioned thermal insulator functioning to define the sidewalls for said cyclic flow channel and said peripherally positioned thermal insulators functioning to form a sidewall for said helium reservoir.

10. A cyclically operating thermal regenerator according to claim 8 wherein helium in said reservoir is at a pressure above 5 atmospheres at temperatures below l6 K and said plate has between 30 to percent open area.

11. A cyclically operating thermal regenerator according to claim 9 wherein said metallic plates are aluminum and said thermal insulators are a thermoplastic. 

1. In a cyclically operating thermal regenerator for operation at temperatures below 16* K wherein heat from a fluid stream flowing in a first direction is received and stored for a finite period by a helium reservoir prior to transfer of said heat to a fluid stream flowing in the opposite direction, the improvement comprising means for controlling the pressure of helium within said reservoir by varying the quantity of helium in said reservoir as the helium temperature changes within said reservoir.
 2. A cyclically operating thermal regenerator according to claim 1 wherein said helium reservoir is situated within a housing externally located relative to said fluid stream flow channel, said thermal contact between said helium reservoir and said fluid streams being effectuated by a high thermal conductivity material protruding through the walls of said helium reservoir housing and extending into both said helium reservoir and said fluid streams flow channel.
 3. A cyclically operating thermal regenerator according to claim 1 wherein the helium within said reservoir is at a pressure above 5 atmospheres and at a temperature of below 16* K.
 4. A cyclically operating thermal regenerator according to claim 2 wherein said high thermal conductivity material is selected from the group consisting of aluminum, copper, and sapphire, said material being a porous structure disposed substantially uniformly in said fluid streams flow channel to assure uniform distribution of flow at any cross-section of said flow channel.
 5. A cyclically operating thermal regenerator according to claim 2 wherein said thermal contact between the reservoir and the flow streams is effectuated by a plurality of porous metallic plates spaced in insulated relationship with respect to one another within the path of said fluid streams at a generally perpendicular attitude to the direction of fluid flow.
 6. A cyclically operating thermal regenerator for operation at temperatures below 16* K comprising a channel for the cyclic flow of a fluid refrigerant therethrough, a reservoir containing a gaseous medium, said reservoir being positioned adjacent said flow channel to sequentially receive, store and transmit heat to said fluid refrigerant during a complete flow cycle of said fluid refrigerant, means for controlling the pressure of the gaseous medium within said reservoir by varying the quantity of said medium in said reservoir as the temperature within said reservoir changes, and a plurality of spaced-apart, high conductivity metallic elements spaced in insulated relationship with respect to one another, said elements forming an apertured obstruction within said refrigerant flow stream and protruding through the sidewalls separating said flow channel from said reservoir to extend into said gaseous medium.
 7. A cyclically operating thermal regenerator for operation at temperatures below 16* K comprising a channel for the cyclic flow of a refrigerant therethrough, a generally stagnant helium heat reservoir positioned adjacent said channel, and a plurality of porous, high heat conductivity elements disposed within said cyclic flow channel and extending into said helium reservoir to conduct heat between said Reservoir and said refrigerant upon the existence of a thermal gradient between said helium and said refrigerant.
 8. A cyclically operating thermal regenerator according to claim 7 wherein said refrigerant cyclically flows through a plurality of channels and said helium reservoir is disposed on opposite sides of each of said channels.
 9. A cyclically operating thermal regenerator according to claim 7 wherein said heat conductivity elements are a plurality of porous metallic plates disposed at a perpendicular attitude relative to the refrigerant flow, adjacent of said plates being separated both by peripherally positioned thermal insulators and by interiorly positioned thermal insulators, said interiorly positioned thermal insulator functioning to define the sidewalls for said cyclic flow channel and said peripherally positioned thermal insulators functioning to form a sidewall for said helium reservoir.
 10. A cyclically operating thermal regenerator according to claim 8 wherein helium in said reservoir is at a pressure above 5 atmospheres at temperatures below 16* K and said plate has between 30 to 60 percent open area.
 11. A cyclically operating thermal regenerator according to claim 9 wherein said metallic plates are aluminum and said thermal insulators are a thermoplastic. 